1. Field of the Invention
The present invention relates to an image forming apparatus for forming a copy by transferring an image formed on a recording medium such as a photosensitive drum onto a transfer material.
2. Description of the Prior Art
In a method of separating a transfer material from a photosensitive body, a high AC voltage sufficient to allow separation of the transfer material from the surface of the photosensitive body is applied to eliminate the charge of the transfer material. This method is subject to a problem of re-transfer wherein toner transferred onto the transfer material is re-transferred to the surface of the photosensitive body due to over-elimination of the transfer material charge. In order to prevent this problem, a DC bias voltage is superposed on the AC voltage, or the DC bias voltage is switched between a predetermined zone from the leading edge of the transfer material and the remaining zone thereof.
FIG. 3 shows the effect of the DC bias voltage on the separation of the transfer material or re-transfer of the toner. As can be seen from FIG. 3, within a DC bias voltage range in which toner re-transfer is unlikely to occur, defective separation of the transfer material is likely to occur.
In view of the above, in the method wherein a DC bias voltage is superposed on the AC voltage, in order to simultaneously perform satisfactory separation of the transfer material and prevent re-transfer of toner, a voltage intermediate between a DC application voltage optimal for separation of the transfer material and a DC application voltage optimal for prevention of toner re-transfer has been applied. With this method, however, separation performance of the transfer material or prevention of toner re-transfer is adversely affected by changes in environmental factors or contamination of the residual charge eliminator.
On the contrary, the method of strongly charge-eliminating only the leading end of the transfer material is subject to the following problem. That is, in order to assure reliable separation of the leading edge, charge elimination must be performed by a high voltage exceeding a predetermined zone of the transfer material, e.g., a zone 20 mm or more from the leading edge of the transfer material. However, in the associated conventional technique, the ON time of the separation charge eliminator is fixed and cannot be changed.
FIGS. 1A and 1B are diagrams for explaining the above problem. FIG. 1A is a diagram illustrating the relationships between a transfer material, an image on the transfer material, and a switching timing of charge eliminating voltage. More specifically, FIG. 1A shows a leading edge X, a trailing edge Y of the transfer material, a non-image zone X'-A of the transfer material on which no image is formed, an image zone A-Y' of the transfer material on which an image is formed, a zone X"-P in which a high charge eliminating voltage is applied, and a zone P-Y" in which an intermediate (middle) charge eliminating voltage is applied. A point P is at a distance of 20 mm or more from the leading edge X. In the case of FIG. 1A, even if a zone P'-A is strongly charge-eliminated, toner re-transfer will not be caused since no toner is present in this zone. However, as shown in FIG. 1B, when the image on the transfer material X-Y is formed in a zone B-Y', since the high charge eliminating voltage application zone X"-P is fixed, a zone B-P' of the image on the transfer material is strongly charge-eliminated. In this zone, re-transfer easily occurs, and stable image quality cannot be obtained.
The method of charge eliminating only the leading end of the transfer material is subject to the following problem. That is, in order to assure separation of the leading end of the transfer material, the separation charge eliminator must exceed a zone 20 mm or more from the leading end of the transfer material. However, the ON interval of the separation charge eliminator is conventionally fixed, and cannot be changed.
FIGS. 1A and 1B are used again to explain the above problem. FIG. 1A shows the relationships of a transfer material, an image formed on the transfer material, and the timing of the charge eliminating voltage. Thus, FIG. 1A shows a leading edge X of the transfer material, a trailing edge Y of the transfer material, a non-image zone X'-A of the transfer material in which no image is formed, an image zone A-Y' of the transfer material in which an image is formed, a charge eliminating voltage application zone X"-P, and a charge eliminating voltage non-application zone P-Y". A point P is at a distance of 20 mm or more from the leading edge X. In the case of FIG. 1A, no toner re-transfer occurs in the zone P'-A since no toner image is formed in this zone. However, when the image on the transfer material X-Y is formed in the zone B-Y' as shown in FIG. 1B, since the charge eliminating voltage application zone X"-P is fixed, charge elimination is performed in the image zone B-P' on the transfer material. In this zone, toner re-transfer and data drop occur, and stable image quality cannot be obtained.
Conventional copying machines mainly adopt the electrostatic separation method for separating a transfer material from a photosensitive body after an electrostatic latent image is formed on the photosensitive body and the resultant toner image is transferred onto the transfer material. Thus, a high AC voltage is applied to the transfer material so as to eliminate the charge of the transfer material and to allow its separation from the surface of the photosensitive body.
However, in this method, the charge of the transfer material is excessively eliminated in accordance with the value of the superposed AC voltage, so that toner which has been transferred onto the transfer material is re-transferred to the surface of the transfer material. In order to prevent such re-transfer of toner, a method of superposing a DC bias voltage to the AC voltage is known. However, although the value of the DC bias voltage applied is kept constant, the area of the image to be reproduced, i.e., the amount of toner to be transferred is not constant. For this reason, defective separation of the transfer material or toner re-transfer still occurs.
The above problem may be attributed to the various area ratios (to be referred to an image area ratio hereinafter) of an image zone (information zone) of an original to the total area thereof. In normal document originals, the image area ratio is 10% or less. However, in digital copying machines, photographs can also be reproduced with high quality by the dither method. In such photograph originals, the image area ratio is as high as 50 to 80%.
FIG. 2 shows the influence of the image area ratio and the DC bias voltage on the separation of the transfer material and toner re-transfer. The experiment conditions to obtain the results shown in FIG. 2 were as follows.
______________________________________ Type of photosensitive Amorphous Si photosensitive body: body Surface potential of Dark potential V.sub.D = +400 V photosensitive body: Bright potential V.sub.L = +50 V AC voltage after 5.0 kVrms separation charge elimination: ______________________________________
In FIG. 2, a hatched region indicates a region wherein separation of the transfer material can be performed without causing toner re-transfer. Referring to FIG. 2, when an original image is entirely white, the DC bias voltage for allowing separation is 0.5 to 1.8 kV. When the applied voltage is lower than the lower limit, the transfer material is electrostatically attracted to the photosensitive body by residual charge on the transfer material and satisfactory separation cannot be performed. On the contrary, when the applied voltage is higher than the upper limit, too many positive coronas are generated to cause excessive charge elimination. Then, the transfer material is charged to the opposite polarity. The transfer material is then electrostatically attracted to the photosensitive body, not allowing satisfactory separation again. When the original image is entirely black, the separable voltage range is -1.0 to 0.9 kV.
When the image is a black image, the DC bias voltage for separation can be lower than that in the case wherein the image is a white image for the following reason. When the original is a black image, a relatively large amount of toner is present between the photosensitive body and the transfer material. For this reason, the gap between the transfer material and the photosensitive body is wide, and the electrostatic attraction force acting between the transfer material and the photosensitive body is smaller than in the case of a white image.
As can be seen from FIG. 2, an optimal DC bias voltage changes in accordance with changes in image area ratio. Thus, although the optimal DC bias voltage must be able to change in accordance with an image area ratio of each original, the DC bias voltage is fixed at an intermediate voltage in conventional apparatuses. For this reason, in the case of originals wherein the image area ratio is extremely high or low, defective separation and/or toner re-transfer cannot be prevented.
In view of this problem, it has been proposed to detect the surface potential or the toner attachment amount on the photosensitive body and to control the DC separation voltage in accordance with the detected surface potential or toner attachment amount. However, in these methods, it is difficult to determine a DC voltage precisely corresponding to the image area ratio of the original and the detection apparatus becomes complex and expensive.